Polarization properties of light are used in a variety of optical applications (products and methods) ranging from flat panel liquid-crystal displays (LCDs) to microscopy, metallurgy inspection and optical communication. Light generated by most light sources does not have a specific polarization, and typical polarization selection is done using polarizers of various types. The selection of a particular polarization using a polarizer comes at the cost of energy loss; approximately 50% of impinging light may be lost when using a simple passive (non-emissive) polarizer to provide polarized illumination using a non-polarized light source. This problem is of particular significance in backlight systems for LCD displays, where energy saving is an important factor. The problem is further intensified in mobile devices (laptops, cellphones, cameras, etc.) where battery life is a crucial factor.
Relatively efficient polarization selection of light emitted by an unpolarized light source can be achieved by locating complex passive (non-emissive) polymer films with special surfaces in the optical path of the emitted light. These films may recycle some of the light and thus enhance the transmission of light of the desired polarization. Recycling of light is based on reflecting light components of unwanted polarization onto a reflecting surface, thereby producing multiply reflected light components which depolarize after subsequent reflections, and thus at least some light components are transmitted after each reflection. However, such passive systems are complex and expensive to produce, as multiple (even tens) of layers are required for efficient recycling. Another “passive” approach to recycling a backlight output through a polarizer uses a reflective nanowire grid polarizer (Ge, Zhibing and Wu, Shin-Tson. “Nanowire grid polarizer for energy efficient and wide-view liquid crystal displays.”, Applied Physics Letters, 93, 121104, 2008).
Passive approaches as described above complicate the design of a backlight system and are expensive. They are also inactive in enhancing the quality of the color gamut of the emitted light, because they are wavelength dependent. In fact, the need to preserve the color gamut of the original backlight complicates further the layer structure of backlight system.
Anisotropic (elongated) nanoparticles such as nanorods (also at times referred to herein as “rods”) are known as being capable of providing polarized emission. This is also described in WO 2010/095140 assigned to the assignee of the present application.
Some nanorod systems providing polarized emission are described in the following publications:
X. Peng et al., “Shape control of CdSe nanocrystals”, Nature 404, 59-61, 2000 describes colloidal based semiconductor core (without shell) CdSe nanorods embedded in a polymer. Nearly full polarization can be obtained from single rods.
T. Mokari and U. Banin, “Synthesis and properties of CdSe/ZnS rod/shell nanocrystals”, Chemistry of Materials 15 (20), 3955-3960, 2003 describes the emission enhancement of rods by growing a shell on the rod structure.
D. V. Talapin, et al, “Seeded Growth of Highly Luminescent CdSe/CdS Nanoheterostructures with Rod and Tetrapod Morphologies”, Nano Letters 7 (10), pp 2951-2959, 2007 describes a quantum yield improvement achieved for seeded nanorod particles.
C. Carbone et al, “Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach”. Nano Letters, 7 (10), pp 2942-2950, 2007 describes a dipole pattern emission of seeded rods, i.e. emission emanating from the rod center rather than its tips.